Backdrop CMS is essentially a fork of early Drupal 8 intended to minimize the costs of transitioning from Drupal 7, partly by reducing the frequency and scope of architecture changes, thus increasing backward compatibility. The two development communities differ in size, but they understandably have many of the same members and skills, just as the two projects still share some code. Consequently, there is ongoing collaboration between the two groups — including core code improvements (e.g., optimization of theme registry and INFO files), contrib module patches, and security issue information and release synchronization. An insightful discussion with the principals is in the DrupalEasy podcast episode #168.